Limonin is a limonoid, and a bitter, white, crystalline substance found in citrus and other plants. It is also known as limonoate D-ring-lactone and limonoic acid di-delta-lactone. Chemically, it is a member of the class of compounds known as furanolactones.

Limonin is enriched in citrus fruits and is often found at higher concentrations in seeds, for example orange and lemon seeds. Limonin is also present in plants such as those of theDictamnus genus.